Cryogenic separation method and apparatus

ABSTRACT

A method and apparatus for separating a mixture, for example air, within a cryogenic rectification plant that utilizes a banked heat exchanger arrangement. In such arrangement, a lower pressure heat exchanger is used to cool part of the mixture and a higher pressure heat exchanger is used to heat one or more pumped liquid streams composed of separated nitrogen-rich and oxygen-rich fractions and thereby produce pressurized product streams. A boosted pressure stream, that can be part of the air, is utilized to supply most of the heat exchange duty in the higher pressure heat exchanger. In addition, a heat exchange stream, that can also be part of the mixture, can be partially cooled in the higher pressure heat exchanger and then further cooled in the lower pressure heat exchanger to decrease the warm end temperature difference of the higher pressure heat exchanger and therefore, the required refrigeration for the plant.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus in which amixture comprising nitrogen and oxygen is separated into nitrogen-richand oxygen-rich fractions through cryogenic rectification and one ormore pressurized product streams are produced by pumping and heating oneor more liquid streams composed of one of the nitrogen-rich oroxygen-rich product fractions. More particularly, the present inventionrelates to such a method and apparatus utilizing a banked arrangement ofheat exchangers in which a heat exchange stream is partially cooledwithin the higher pressure heat exchanger utilized in heating the one ormore pumped liquid streams and then further cooled within the lowerpressure heat exchanger to reduce warm end temperature difference withinthe higher pressure heat exchanger and therefore, the amount ofrefrigeration required to be imparted in connection with the cryogenicrectification.

BACKGROUND OF THE INVENTION

Nitrogen and oxygen containing mixtures, most commonly air, areseparated into nitrogen-rich and oxygen-rich fractions and othercomponent fractions found in air, for example, argon, by cryogenicrectification. In cryogenic rectification, the mixture is compressed andthen purified to remove higher boiling contaminants such as carbondioxide, water vapor and hydrocarbons. The resulting compressed andpurified stream is then cooled to a temperature suitable fordistillation. The distillation produces nitrogen-rich and oxygen-richfractions of the air and potentially other fractions that can be takenas both liquid and gaseous products. There are different distillationcolumn arrangements that are used for such purposes. For example, thedistillation columns can consist of a higher pressure column and a lowerpressure column thermally linked by a heat exchanger to vaporize anoxygen-rich liquid column bottoms of the lower pressure column and tocondense a nitrogen-rich vapor column overhead of the higher pressurecolumn.

Where there exists a demand for large amounts of high pressure oxygen ina gaseous form, in for example, gasification, the oxygen is oftenproduced by pumping a stream of oxygen-rich liquid to pressure and thenheating the liquid through indirect heat exchange with a boostedpressure stream that is typically part of the air to be distilled. Inthe column arrangement discussed above, the stream of oxygen-rich liquidis composed of the oxygen-rich liquid column bottoms of the higherpressure column. The boosted pressure air will be liquefied as a resultof the heat exchange and introduced into the higher pressure column, thelower pressure column or divided between the columns after having beenreduced to a pressure suitable for introduction into such columns. Inaddition to oxygen, high pressure nitrogen can also be a desiredproduct, particularly in gasification applications. Such high pressurenitrogen can be produced by pumping a stream of the condensed,nitrogen-rich vapor and heating the resulting liquid through heatexchange with the boosted pressure stream.

It is to be noted that the heating of the pumped streams can be carriedout in a banked arrangement of heat exchangers having a higher pressureheat exchanger for the heating of the pumped liquid and a lower pressureheat exchanger, having a lower maximum allowable working pressure thanthe higher pressure heat exchanger for cooling at least part of the air.In this regard, although as discussed above, the boosted pressure streamis commonly composed of air, other fluids can be used. For example, inU.S. Pat. No. 4,345,925, a fluid such as argon is compressed and thenliquefied as a result of the indirect heat exchange occurring in thehigher pressure heat exchanger. The resulting liquid then serves invarious heat exchange functions related to the distillation columns. Inparticular, the fluid is vaporized and superheated in the lower pressureheat exchanger and then recompressed prior to its introduction into thehigher pressure heat exchanger. Thus, the fluid circulates in a heatexchange loop that involves the heating of the pressurized oxygen-richliquid.

An example of a cryogenic rectification plant in which air serves as theboosted pressure fluid can be found in United States Patent Application,Publication Number 2008/0307828. In this application, air is compressedand purified. Part of the resulting compressed and purified air iscooled within the lower pressure heat exchanger and then introduced intothe higher pressure column as a main air stream. The higher pressurecolumn is thermally linked to a lower pressure column by a condenserreboiler. The condenser reboiler condenses nitrogen-rich vapor columnoverhead of the higher pressure column against vaporizing oxygen-richliquid column bottoms formed in the lower pressure column. Part of anoxygen-rich liquid stream composed of the oxygen-rich liquid columnbottoms of the low pressure column can be pumped to form a pumped liquidoxygen stream and part of the condensed nitrogen-rich vapor can also bepumped to produce a pumped liquid nitrogen stream. The pumped liquidoxygen stream and the pumped liquid nitrogen stream are then heated in ahigher pressure heat exchanger through indirect heat exchange withboosted pressure stream that is formed by compressing another part ofthe compressed and purified air in a booster compressor. The boostedstream is liquefied, expanded and introduced into the higher and lowerpressure columns. A thermal balancing stream composed of a wastenitrogen stream produced in the lower pressure column is introduced intoboth the lower pressure heat exchanger and the higher pressure heatexchanger to inhibit warm end losses of refrigeration by such heatexchangers and also to decrease the difference in temperatures betweenthe boosted pressure stream and the main air stream at the cold end ofsuch heat exchangers.

In any cryogenic rectification plant, refrigeration must be imparted forsuch reasons as warm end losses in the heat exchangers, heat leakageinto the plant and to produce liquids. In the higher pressure heatexchanger, the magnitude of the warm end temperature difference betweenthe boosted stream being cooled versus the vaporized liquid stream orstreams being heated represents a loss of such refrigeration. In orderto overcome such loss of refrigeration, more refrigeration must beintroduced into the plant. In the published patent application,discussed above, this refrigeration can be produced by introducing yetanother part of the compressed and purified air into a boostercompressor, partially cooling such air in either the higher or the lowerpressure heat exchanger and then, introducing the partially cooledstream into a turboexpander to generate the refrigeration in an exhauststream that can be introduced into the higher pressure column or thelower pressure column. The greater the degree of refrigeration that isrequired for a plant, the greater the energy that will be expanded inthe associated booster compressor. Since, the total energy expenditureis an important consideration in the cost of production in a cryogenicrectification plant, it is desired to minimize energy requirements forthe plant. As will be discussed, among other advantages, the presentinvention provides a method and a cryogenic rectification plant forconducting such a method in which warm end temperature differencesproduced in the higher pressure heat exchanger are reduced to decreasethe refrigeration requirements and therefore, the costs involved inoperating the plant.

SUMMARY OF THE INVENTION

The present invention, in one aspect, provides a method of separating amixture comprising nitrogen and oxygen. In accordance with this aspectof the present invention, a cryogenic rectification process is conductedthat comprises compressing, purifying, cooling and distilling themixture into oxygen into nitrogen-rich and oxygen-rich fractions,imparting refrigeration into the cryogenic rectification process andproducing at least one pressurized product stream by pumping and heatingat least part of one of the nitrogen-rich and oxygen-rich fractions in aliquid state.

The cryogenic rectification process is conducted so as to produce aboosted pressure stream and a heat exchange stream and the cryogenicrectification process utilizes a banked heat exchanger arrangementhaving a lower pressure heat exchanger for the cooling of at least partof the mixture and a higher pressure heat exchanger for heating the atleast part of the one of the nitrogen-rich and oxygen-rich fractions.The boosted pressure stream and the heat exchange stream are introducedinto the higher pressure heat exchanger in indirect heat exchange withthe at least part of the one of the nitrogen-rich and oxygen-richfractions. The heat exchange stream is partially cooled in the higherpressure heat exchanger, thereby decreasing a warm end temperaturedifference within the higher pressure heat exchanger and therefore, therefrigeration required to be imparted to the cryogenic rectificationprocess. The boosted pressure stream is cooled within the higherpressure heat exchanger and the heat exchange stream is further cooledin the lower pressure heat exchanger.

The reduction of the warm end temperature difference within the higherpressure heat exchanger reduces refrigeration losses to in turn decreasethe amount of refrigeration that is required for the cryogenicrectification process. The decrease in refrigeration requirementtranslates into a reduction in the power consumption of the plant due tolower compression requirements for generating the refrigeration in thefirst instance. As used herein and in the claims, the term “partiallycooled” means cooled to a temperature intermediate the temperatures thatcan be achieved at the warm and cold ends of a heat exchanger. The term,“fully cooled” as used herein and in the claims means cooled to atemperature of the cold end of a heat exchanger and “fully warmed” meanswarmed to a temperature of the warm end of the heat exchanger.

The cryogenic rectification process can generate a nitrogen-rich vaporstream that is divided into two nitrogen-rich vapor streams that arefully warmed within the higher pressure heat exchanger and the lowerpressure heat exchanger so as to balance cold end temperatures of thehigher pressure heat exchanger and the lower pressure heat exchanger.The mixture can be air and in such case, a feed stream composed of theair, after having been compressed and purified, is divided into a firstsubsidiary compressed air stream; a second subsidiary compressed airstream and a third subsidiary compressed air stream. The firstsubsidiary compressed air stream is fully cooled within the lowerpressure heat exchanger, at least part of the second subsidiarycompressed air stream is further compressed to form the boosted pressurestream that forms a liquid air stream after having been fully cooledwithin the higher pressure heat exchanger and the heat exchange streamis the third subsidiary compressed air stream. A first part of thesecond subsidiary compressed air stream can be further compressed toproduce the boosted pressure air stream and a second part of the thirdsubsidiary compressed air stream can be further compressed to a pressurebelow that of the boosted pressure air stream and is thereafter, yetfurther compressed, partially cooled within the higher pressure heatexchanger and introduced into a turboexpander to produce an exhauststream. The exhaust stream is introduced into the distillation column toimpart at least part of the refrigeration into the cryogenicrectification process.

In a specific embodiment of the present invention, the mixture isdistilled in a higher pressure column operatively associated in a heattransfer relationship with a lower pressure column by acondenser-reboiler configured to condense a higher pressurenitrogen-rich column overhead stream, removed from the higher pressurecolumn, by reboiling an oxygen-rich liquid column bottoms of the lowerpressure column. The first subsidiary compressed air stream and thethird subsidiary compressed air stream, after having been fully cooled,are introduced into the higher pressure column. The liquid air stream isexpanded and introduced into at least one of the higher pressure columnand the lower pressure column. A crude liquid oxygen stream composed ofa liquid column bottoms of the higher pressure column is subcooled,reduced in pressure to that of the lower pressure column and introducedinto the lower pressure column for further refinement. First and secondparts of a high pressure nitrogen-rich liquid stream, formed fromcondensing the higher pressure nitrogen-rich column overhead stream, isused to reflux the higher pressure column and the lower pressure column,respectively. The second of the parts of the higher pressurenitrogen-rich liquid stream is subcooled, reduced in pressure to that ofthe lower pressure column prior to being introduced as the reflux intothe lower pressure column and the crude liquid oxygen stream and thesecond of the parts of the higher pressure nitrogen-rich liquid streamare subcooled through indirect heat exchange with a lower pressurenitrogen-rich column overhead stream withdrawn from the lower pressurecolumn. The at least one liquid stream is one of an oxygen-enrichedstream, composed of the oxygen-rich liquid column bottoms of the lowerpressure column and a third part of the high pressure nitrogen-richliquid stream.

The lower pressure nitrogen-rich column overhead stream can be dividedinto the two nitrogen-rich vapor streams that are utilized to balancethe cold end temperatures of the higher pressure heat exchanger and thelower pressure heat exchanger. The exhaust stream is introduced into thehigher pressure column and the liquid air stream is expanded in a liquidexpander.

In another aspect, the present invention provides an apparatus forseparating a mixture comprising nitrogen and oxygen. In accordance withthis aspect of the present invention, the apparatus comprises acryogenic rectification plant configured to compress, purify, cool anddistill the mixture into nitrogen-rich and oxygen-rich fractions. Thecryogenic rectification plant has at least one pump for pumping at leastpart of a liquid stream composed of one of the nitrogen-rich andoxygen-rich fractions in the liquid state and a banked heat exchangerarrangement having lower pressure heat exchanger configured to cool atleast part of the mixture and a higher pressure heat exchanger in flowcommunication with the at least one pump for heating the at least partof the liquid stream and thereby producing a pressurized product stream.Additionally, a means is provided for producing a boosted pressurestream, a means is provided for producing a heat exchange stream and ameans is provided for imparting refrigeration into the cryogenicrectification plant. The higher pressure heat exchanger is connected tothe boosted pressure stream producing means and the heat exchange streamproducing means and is configured to partially cool the heat exchangestream by indirectly exchanging heat from the heat exchange stream tothe at least part of the liquid stream, thereby decreasing a warm endtemperature difference within the higher pressure heat exchanger andtherefore, the refrigeration required to be imparted to the cryogenicrectification plant and to cool the boosted pressure stream byindirectly exchanging heat from the boosted pressure stream to the atleast part of the liquid stream. The lower pressure heat exchanger isconnected to the higher pressure heat exchanger and is configured tofurther cool the heat exchange stream after having been partially cooledwithin the higher pressure heat exchanger.

The cryogenic rectification plant can also be configured to generate twonitrogen-rich vapor streams and the higher pressure heat exchanger andthe lower pressure heat exchanger are also configured to receive and tofully warm the two nitrogen-rich vapor streams so that cold endtemperatures of the higher pressure heat exchanger and the lowerpressure heat exchanger are balanced. The mixture can be air and in suchcase, the cryogenic rectification plant has a main air compressor and apre-purification unit connected to the main air compressor to purify theair after having been compressed. The boosted pressure producing meanscomprises a booster compressor connected to the pre-purification unitand the lower pressure heat exchanger is also connected to thepre-purification unit so that a feed stream composed of the mixtureafter having been compressed in the main air compressor and purified inthe pre-purification unit is divided into a first subsidiary compressedair stream that is fully cooled in the lower pressure heat exchanger anda second subsidiary compressed air stream that at least in part isfurther compressed in the booster compressor to form the boostedpressure stream and that forms a liquid air stream after having beenfully cooled within the higher pressure heat exchanger. The heatexchange stream producing means comprises the higher pressure heatexchanger also connected to the pre-purification unit so that the feedstream after having been compressed and purified is further divided intoa third subsidiary compressed air stream that forms the heat exchangestream. The booster compressor can be a multi-stage machine. A firstpart of the second subsidiary compressed air stream is discharged from afinal stage of the booster compressor and forms the boosted pressure airstream. The refrigeration imparting means, at least in part, comprises afurther booster compressor connected to an intermediate stage of thebooster compressor to further compress a second part of the secondsubsidiary compressed air stream. The higher pressure heat exchangerconnected to the further booster compressor so that the second part ofthe third subsidiary compressed air stream, after having been furthercompressed, is partially cooled within the higher pressure heatexchanger and a turboexpander is connected to the higher pressure heatexchanger to expand the first part of the second subsidiary compressedair stream and thereby to produce an exhaust stream. The turboexpanderis connected to the distillation column so that the exhaust stream isintroduced into the distillation column.

The cryogenic rectification plant can have a higher pressure column anda lower pressure column to distill the mixture. The higher pressurecolumn is operatively associated in a heat transfer relationship withthe lower pressure column by a condenser-reboiler. At least part of ahigher pressure nitrogen-rich column overhead stream, discharged fromthe higher pressure column, is condensed by reboiling an oxygen-richliquid column bottoms of the lower pressure column. The lower pressureheat exchanger is connected to the higher pressure column so that thefirst subsidiary compressed air stream and the second subsidiarycompressed air stream are introduced into the higher pressure column.The higher pressure heat exchanger is in flow communication with atleast one of the higher pressure column and the lower pressure column sothat the liquid air stream is introduced into at least one of the higherpressure column and the lower pressure column. An expansion device ispositioned between the higher pressure heat exchanger and the at leastone of the higher pressure column and the lower pressure column toexpand the liquid air stream. The higher pressure column is connected tothe lower pressure column so that a crude liquid oxygen stream composedof liquid column bottoms of the higher pressure column is introducedinto the lower pressure column so as to be further refined and first andsecond parts of a high pressure nitrogen-rich liquid stream, formed fromcondensing the higher pressure nitrogen-rich overhead stream, areintroduced into the higher pressure column and the lower pressurecolumn, respectively, as reflux. A subcooler, positioned between thelower pressure column and the lower pressure heat exchanger orincorporated into the lower pressure heat exchanger, is configured tosubcool the crude liquid oxygen stream and the second of the parts ofthe higher pressure nitrogen-rich liquid stream. Expansion valves arelocated between the subcooler and the lower pressure column to expandthe crude liquid oxygen stream and the second of the parts of the higherpressure nitrogen-rich liquid stream prior to their introduction intothe lower pressure column. The subcooler is connected to the lowerpressure column so that a lower pressure nitrogen-rich column overheadstream discharged from the lower pressure column passes in indirect heatexchange with the crude liquid oxygen stream and the second of the partsof the higher pressure nitrogen-rich liquid stream and the at least oneliquid stream is an oxygen-enriched stream, composed of the oxygen-richliquid column bottoms of the lower pressure column or a third part ofthe higher pressure nitrogen-rich liquid stream.

The higher pressure heat exchanger and the lower pressure heat exchangercan be connected to the subcooler so that the lower pressurenitrogen-rich column overhead stream divides into the two nitrogen-richvapor streams that are utilized to balance the cold end temperatures ofthe higher pressure heat exchanger and the lower pressure heatexchanger. Further the turboexpander is connected to the higher pressurecolumn so that the exhaust stream is introduced into the higher pressurecolumn and the expansion device is a liquid expander.

BRIEF DESCRIPTION OF THE DRAWING

While the specification concludes with claims distinctly pointing outthe subject matter that Applicant regards as his invention it isbelieved that the invention will be better understood when taken inconnection with the accompanying sole FIGURE that illustrates anapparatus for carrying out a method in accordance with the presentinvention.

DETAILED DESCRIPTION

With reference to the sole FIGURE, a cryogenic rectification plant 1 inaccordance with the present invention is illustrated that is designed tocryogenically rectify air or another mixture that contains nitrogen andoxygen into nitrogen and oxygen fractions as will be discussed below.For example, the feed to a cryogenic rectification plant of the presentinvention could be derived from another air separation plant and assuch, the feed might be richer in oxygen in a concentration that ishigher than air. Furthermore, although the present invention isillustrated in connection with a double column system having a higherpressure column operatively associated with a lower pressure column in aheat transfer relationship by virtue of a condenser reboiler to condensea nitrogen-rich vapor column overhead in the higher pressure columnagainst vaporizing an oxygen-rich liquid column bottoms of the lowerpressure column, the invention is not limited to such a columnarrangement. In this regard, the present invention has application toany cryogenic rectification plant employing a banked arrangement of heatexchangers in which a liquid stream enriched in a separated component,typically, nitrogen and oxygen, is pumped and then heated in a higherpressure heat exchanger to form a pressurized product either as a highpressure vapor or as a supercritical fluid.

A feed air stream 10 is compressed in a main compressor 12. Afterremoval of the heat of compression by a first after-cooler 14, feed airstream 10 is purified within a pre-purification unit 16 to produce acompressed and purified air stream 17. Here it is appropriate to pointout that although the after-cooler 14 is shown as a separate unit, suchcompressors as main compressor 12 could be multiple stage machines withintercoolers and an after-cooler installed by the manufacturer as partof the compressor. As such, the after-cooler might not be a separateunit as illustrated and instead, could be part of the compressor itself.The foregoing comments would be equally applicable to any of thecompressor and after-cooler arrangements discussed hereinafter.Pre-purification unit 16, as well known to those skilled in the art cancontain beds of adsorbent, for example alumina or carbon molecularsieve-type adsorbent to adsorb the higher boiling impurities containedwithin the air and therefore feed air stream 10. For example such higherboiling impurities as well known would include water vapor and carbondioxide that will freeze and accumulate at the low rectificationtemperatures contemplated by apparatus 1. In addition, hydrocarbons canalso be adsorbed that could collect within oxygen-rich liquids andthereby present a safety hazard.

A first subsidiary compressed air stream 18 is produced from a firstpart of the compressed and purified air stream 17. A booster compressor20 is in flow communication with purification unit 16 to compress asecond subsidiary compressed air stream 22 formed from a second part ofthe compressed and purified air stream 17 and a second after-cooler 23is provided that is connected to booster compressor 20 to remove theheat of compression from the second subsidiary compressed air stream 22after having been further compressed. This forms a boosted pressurestream 24 having a higher pressure than the first subsidiary compressedair stream 18. It is to be noted that main air compressor 10 and boostercompressor 20 are shown as single units. However, as is known in theart, two or more compressors can be installed in parallel to form eitherthe main air compressor 10 or the booster compressor 20. The twocompressors can be of equal size or unequal size. For example, thecapacity can be split 70/30 or 60/40 in order to better match customerdemand. Typically, second subsidiary compressed air stream 22 will havea flow that ranges from between about 25 percent and about 40 percent ofthe flow of the compressed air stream 17.

A higher pressure heat exchanger 26 is connected to second after-coolers23 and 101 and a lower pressure heat exchanger 28 is in flowcommunication with purification unit 16 to receive the first subsidiarycompressed air stream 18. Both the higher pressure heat exchanger 26 andthe lower pressure heat exchanger 28 are preferably of brazed aluminumconstruction and consist of layers of parting sheets separated by sidebars to produce flow passages for the streams to be heated and cooled.Each of the flow passages are provided with fins as well known in theart to enhance the surface area for heat transfer within said heatexchangers. The higher pressure heat exchanger 26 is so named due to thefact that it has a higher maximum allowable working pressure as comparedwith lower pressure heat exchanger 28. The higher pressure heatexchanger 26 is configured to fully cool the boosted pressure stream 24to produce a liquid air stream 30 and the lower pressure heat exchanger28 is configured to fully cool the first subsidiary compressed airstream 18 to produce a main feed air stream 32. As can be appreciated,other types of heat exchangers could be used, for example, higherpressure heat exchanger 26 could be spiral wound, printed circuit or ofstainless steel plate-fin construction. Moreover, although each of thehigher pressure heat exchanger 26 and the lower pressure heat exchanger28 are illustrated as single units, in practice, each could consist ofseveral heat exchangers linked together in parallel.

The lower pressure heat exchanger 28 will have a larger cross-sectionalarea for flow and a large total volume than the higher pressure heatexchanger 26. Typically the average density of the higher pressure heatexchanger 26 will be greater than the lower pressure heat exchanger 28where density is the empty weight divided by volume. A typical densityis about 1000 kg/m³. A typical working pressure of the higher pressureheat exchanger 26 is about 1200 psig and greater.

An air separation unit 34 is provided that has a higher pressure column36 operatively associated with a lower pressure column 38 in a heattransfer relationship by means of a condenser-reboiler 50. Optionally,air separation unit 34 can also include an argon column that isconnected to lower pressure column 38 for producing an argon product. Itis understood that each of the higher pressure column 36 and the lowerpressure column 38 contain liquid-vapor mass transfer elements such assieve trays or packing, either random or structured. Such elements aswell known in the art enhance liquid-vapor contact of liquid and vaporphases of the mixture to be separated in each of such columns forrectification purposes. The rectification of the air within suchdistillation columns produces nitrogen-rich and oxygen-rich fractions ofthe air as nitrogen-rich column overhead of the higher pressure column36 and a nitrogen-rich column overhead of the lower pressure column 38,at of course a lower pressure than the nitrogen-rich vapor produced inthe higher pressure column 36 and an oxygen-rich liquid as a liquidcolumn bottoms of the lower pressure column 38. As will be discussed,streams of these fractions can be directly taken as products orcondensed and/or pressurized and warm to form products of the cryogenicrectification plant 1.

The liquid air stream 30 is expanded to a pressure suitable for itsintroduction into higher pressure column 36 by way of a liquidturboexpander 40. Energy from liquid turboexpander 40 can be recoveredand thus the liquid turboexpander can generate part of the refrigerationrequirement for the cryogenic rectification plant 1. Alternatively, anexpansion valve can be used (or a combination of the two). After havingbeen expanded, liquid air stream 30 is divided into a first subsidiaryexpanded stream 42 and a second subsidiary expanded stream 44. Secondsubsidiary expanded stream 44 is expanded by an expansion valve 46 topressure suitable for its introduction into lower pressure column 38 asa further expanded stream 47. Thus, both first and second subsidiaryexpanded streams 42 and 44 are introduced into intermediate locations ofhigher and lower pressure columns 36 and 38, respectively at pointsthereof that would match the composition of the mixture being separatedin the columns. It is understood, however, that embodiments of thepresent invention are possible in which the liquid air stream 30 isintroduced into either the higher pressure column 36 or the lowerpressure column 38.

The rectification of the air within higher pressure column 36 produces acrude liquid oxygen column bottoms and a nitrogen-rich vapor columnoverhead. Part of a nitrogen-rich vapor column overhead stream 48 iscondensed in condenser-reboiler 50 against vaporizing an oxygen-richcolumn bottoms that is produced by the rectification occurring in thelower pressure column. In this regard, such rectification also produces,within lower pressure column 38, a nitrogen-rich vapor column overhead.The resultant condensation produces a nitrogen-rich liquid stream 52.First part 54 of nitrogen-rich liquid stream 52 is returned to higherpressure column 36 as reflux. A second part 56 is subcooled within asubcooling unit 29 that as illustrated is an integral part of lowerpressure heat exchanger 28 by provision of suitable passages therein.However, as would be known to those skilled in the art, subcooling unit29 could in fact be a separate heat exchanger or separate heatexchangers operating in parallel. The resulting subcooled liquidnitrogen stream 58 is then further optionally subdivided into parts 60and 62. Part 60 of the subcooled liquid nitrogen stream 58 is thenexpanded within an expansion valve 64 to a pressure suitable for itsintroduction to lower pressure column 38 and then introduced into lowerpressure column 38 as reflux. Part 62 of subcooled liquid nitrogenstream 58 can be taken as an optional liquid product. The distillationof the air produces a crude liquid oxygen column bottoms, also known inthe art as kettle liquid, in the higher pressure column 36. A crudeliquid oxygen stream 58 composed of the crude liquid oxygen columnbottoms is also subcooled within the subcooling unit 29 incorporatedinto lower pressure heat exchanger 28 and then expanded in an expansionvalve 68 to be introduced into lower pressure column 38 for furtherrefinement.

A nitrogen-rich vapor stream 70 can be removed from the top of lowerpressure column 38 that consists of the lower pressure nitrogen-richvapor column overhead produced as a result of the distillation occurringwithin the lower pressure column 38. Although not illustrated, as knownin the art, a waste nitrogen stream could also be removed below the topof low pressure column 38 in order to maintain the purity ofnitrogen-rich vapor stream 70 if the same were required in a resultingproduct. Since this has not been done in the illustrated embodiment,nitrogen-rich vapor stream 70 is not of typical product purity in thatit is contaminated with higher amounts of oxygen than a nitrogen productstream. The nitrogen-rich vapor stream 70 is then subdivided into twosubsidiary nitrogen-rich vapor streams 71 and 72. Subsidiarynitrogen-rich vapor stream 71 is partially warmed in the subcooling unitportion of the lower pressure heat exchanger 28 in order to subcoolsecond part 56 of the nitrogen-rich liquid stream 56 and the crudeliquid oxygen stream 58. The subsidiary nitrogen-rich vapor stream 71 isthen fully warmed within lower pressure heat exchanger 28 to form awaste nitrogen stream 73. As illustrated, the waste nitrogen stream 73can be used to regenerate adsorbent beds within the pre-purificationunit 16 in a manner known in the art. The subsidiary nitrogen-rich vaporstream 72 is fully warmed in the higher pressure heat exchanger to formwaste nitrogen stream 74. The flow rates of these streams are selectedto balance the cold end temperature difference of the higher pressureheat exchanger 26 and the lower pressure heat exchanger 28. In thisregard, if the temperature of liquid air stream 30 were too high, theliquid produced by the liquid turboexpander 40 after expansion to columnpressure will produce too much vapor and as result, the desireddistillation will not occur within the distillation columns.

An oxygen-rich liquid stream 75, composed of the oxygen-rich liquidcolumn bottoms of lower pressure column 38, can be removed from thelower pressure column 38. A first part 76 of the oxygen-rich liquidstream 75 can be pressurized by a pump 78 to produce a pumped liquidoxygen stream 80. A second part 82 of the oxygen-rich liquid stream 75can optionally be taken as a product. Pumped liquid oxygen stream 80,the two nitrogen-rich vapor streams 71 and 72, and as will be discussed,second subsidiary nitrogen vapor stream 84 and pumped liquid nitrogenstream 92 constitute return streams of the air separation unit 34 thatare used to cool the incoming air within higher pressure heat exchanger26 and lower pressure heat exchanger 28. As illustrated, optionally,nitrogen-rich vapor stream 48 can be divided into first and secondsubsidiary nitrogen vapor streams 82 and 84. First subsidiary nitrogenvapor stream 82 is introduced into condenser reboiler 50 and secondsubsidiary nitrogen vapor stream 84 is fully warmed within the lowerpressure heat exchanger 28 and forms a nitrogen product stream 86. Athird portion 88 of the nitrogen-rich liquid stream 52 can optionally bepumped in a pump 90 to produce a pumped liquid nitrogen stream 92 thatis fully warmed within the higher pressure heat exchanger 26 to producea pressurized nitrogen product stream 94. It is to be noted that ifpressurized nitrogen products were desired at different pressures,pumped liquid nitrogen stream 92 could be subdivided and pumped to thedifferent pressures. Pumped liquid oxygen stream 80 is similarly fullywarmed within higher pressure heat exchanger 26 to produce a pressurizedoxygen product stream 96.

As well known in the art, any cryogenic rectification plant must berefrigerated for such reasons as overcoming warm end heat exchangelosses, heat leakage into the cold box containing the distillationcolumns and to allow for the production of liquid products. In cryogenicrectification plant 1, a part 98 of the second subsidiary compressed airstream 22 is extracted from an intermediate stage of booster compressor20 and then, is further compressed in a booster compressor 100. Part 98of the second subsidiary compressed air stream 22 will typically bebetween about 5 percent and about 20 percent of the flow of thecompressed and purified air being discharged from pre-purification unit16. After removal of the heat of compression in an after-cooler 101,such stream is partially cooled within the higher pressure heatexchanger 26 to produce a partially cooled stream 103 that is introducedinto a turboexpander 104 to produce an exhaust stream 105 that isintroduced into the higher pressure column 36 to impart the requiredrefrigeration into the cryogenic rectification plant 1. As would beknown in the art, this is but one option for imparting refrigerationinto a cryogenic rectification plant. For example, depending upon theproduct make desired, the exhaust stream could be introduced into thelower pressure column 38 or a waste nitrogen stream could be expanded.

As indicated above, the boosted pressure stream 24 is fully cooledwithin the higher pressure heat exchanger 26. This being said,embodiments are possible in which boosted pressure stream 24 is removedprior to the cold end of the higher pressure heat exchanger 26 and assuch, has a warmer temperature. In any event, the purpose of the boostedpressure stream 24 is to provide the major part of the heat transferduty in heating the pumped liquid oxygen stream 80 and the pumped liquidnitrogen stream 92 in producing the pressurized nitrogen product stream94 and the pressurized oxygen stream 96. In this regard, both of thepumped liquid oxygen and nitrogen streams 80 and 92 could be pressurizedto a supercritical pressure and upon heating to supercriticaltemperatures, the resulting pressurized product nitrogen stream 94 andthe pressurized production oxygen stream 96 would be supplied assupercritical fluids. However, the present invention also contemplatesthat such fluids would be pressurized to subcritical pressures and assuch, would be vaporized upon heating to be supplied as high pressurevapor streams. It is to be noted that in place of the boosted pressurestream 24 being derived from air, other fluids could be used such asargon as shown in U.S. Pat. No. 4,345,925, discussed above, in which theboosted pressure stream circulates in a closed heat exchange loop.

In all cases, however, there is a refrigeration loss with respect tosuch boosted pressure streams having a magnitude that increases with thedegree of warm end temperature difference. The warm end temperaturedifference in case of the higher pressure heat exchanger 26 is thedifference, as measured at the warm end thereof, between the averagetemperature of the streams being cooled, namely, boosted pressure stream24 and the part 98 of the second subsidiary compressed air stream 22 andthe average temperature of the streams being warmed, namely, pumpedliquid oxygen and nitrogen streams 94 and 96 and the subsidiarynitrogen-rich vapor stream 72. The greater the degree of such warm endtemperature difference, the greater the amount of refrigeration thatwill be required. Practically, in the case of cryogenic rectificationplant 1, the greater refrigeration requirement would be supplied bybooster compressor 100 and therefore, a higher power consumption by suchcompressor. In order to decrease the warm end temperature difference,the degree of refrigeration required and therefore, the powerconsumption, a third subsidiary compressed air stream 106 is producedfrom part of the compressed and purified air stream 17. Such streamserves as a heat exchange stream that is partially cooled within thehigher pressure heat exchanger 26 and then further cooled within thelower pressure heat exchanger 28 to form a cooled air stream 108 thatcan be combined with first subsidiary compressed air stream 18 withinthe lower pressure heat exchanger 28. The combination of the two streamsmay not be necessarily carried out within the heat exchanger. They canproceed separately to the lower column. The resultant combined stream110 can be further combined with the exhaust stream 105 and introducedinto the higher pressure column as a stream 112.

As would be known, there are other possibilities for forming such a heatexchange stream. For example, a part of the waste nitrogen stream 73could be compressed and then partially cooled within higher pressureheat exchanger 26 in place of the illustrated heat exchange stream 106.Such stream could then be further cooled within the lower pressure heatexchanger 28 and then introduced into an intermediate location of thehigher pressure column 36.

The following Table is a stream summary derived from a calculatedexample of the operation of the cryogenic rectification plant 1.

TABLE Stream Flow Temperature, Pressure, Percent No. kcfh K psiaComposition vapor 17 25,540 283.0 85.45 air 100 18 15,470 283.0 85.45air 100 24 8,076 308.0 1600 air 100 112 17,460 112.0 81.45 air 100 1061,662 283.0 85.45 air 100 30 8,076 99.69 1599 air 0 42 4,038 97.24 85.0air 0 47 4,038 81.5 19.1 air 15.8 103 328.8 178.0 695 air 100 105 328.8101.0 81.95 air 100 62 5.5 77.36 80.9 99.785% N₂ + 0 Ar + O2 86 84.48279.3 78.9 99.785% N₂ + 100 Ar + O₂ 82 5.11 93.63 20.85 99.6% O₂ 0 965,325 300 1160 99.6% O₂ 100 70 20,100 80.11 19.64 99.785% N₂ 100 7315,420 279.3 17.24 99.785% N₂ 100 72 4,684 80.07 19.44 99.785% N₂ 100 744,684 300.0 17.44 99.785% N₂ 100 94 11.4 300.0 947.8 99.785% N₂ + 100Ar + O₂

As would occur to those skilled in the art, although the presentinvention has been discussed with respect to a preferred embodiment,numerous changes, addition and omissions to such embodiment could bemade in accordance with the spirit and scope of the present invention asset forth in the appended claims.

1. A method of separating a mixture comprising nitrogen and oxygen, saidmethod comprising: conducting a cryogenic rectification processcomprising: compressing, purifying, cooling and distilling the mixtureinto oxygen into nitrogen-rich and oxygen-rich fractions, impartingrefrigeration into the cryogenic rectification process and producing atleast one pressurized product stream by pumping and heating at leastpart of one of the nitrogen-rich and oxygen-rich fractions in a liquidstate; the cryogenic rectification process being conducted so as toproduce a boosted pressure stream and a heat exchange stream and thecryogenic rectification process utilizing a banked heat exchangerarrangement having a lower pressure heat exchanger for the cooling of atleast part of the mixture and a higher pressure heat exchanger for theheating the at least part of the one of the nitrogen-rich andoxygen-rich fractions after having been pumped; introducing the boostedpressure stream and the heat exchange stream into the higher pressureheat exchanger in indirect heat exchange with the at least part of theone of the nitrogen-rich and oxygen-rich fractions; partially coolingthe heat exchange stream in the higher pressure heat exchanger, therebydecreasing a warm end temperature difference within the higher pressureheat exchanger and therefore, the refrigeration required to be impartedto the cryogenic rectification process; cooling the boosted pressurestream within the higher pressure heat exchanger; and further coolingthe heat exchange stream in the lower pressure heat exchanger afterhaving been partially cooled in the higher pressure heat exchanger. 2.The method of claim 1, wherein the cryogenic rectification processgenerates a nitrogen-rich vapor stream that is divided into twonitrogen-rich vapor streams that are fully warmed within the higherpressure heat exchanger and the lower pressure heat exchanger so as tobalance cold end temperatures of the higher pressure heat exchanger andthe lower pressure heat exchanger.
 3. The method of claim 1 or claim 2,wherein: the mixture is air; a feed stream composed of the air afterhaving been compressed and purified is divided into a first subsidiarycompressed air stream; a second subsidiary compressed air stream and athird subsidiary compressed air stream; the first subsidiary compressedair stream is fully cooled within the lower pressure heat exchanger; atleast part of the second subsidiary compressed air stream is furthercompressed to form the boosted pressure stream and forms a liquid airstream after having been fully cooled within the higher pressure heatexchanger; and the heat exchange stream is the third subsidiarycompressed air stream.
 4. The method of claim 3, wherein: a first partof the second subsidiary compressed air stream is further compressed toproduce the boosted pressure air stream; a second part of the thirdsubsidiary compressed air stream is further compressed to a pressurebelow that of the boosted pressure air stream and is thereafter, yetfurther compressed, partially cooled within the higher pressure heatexchanger and introduced into a turboexpander to produce an exhauststream; and the exhaust stream is introduced into the distillationcolumn to impart at least part of the refrigeration into the cryogenicrectification process.
 5. The method of claim 4, wherein: the mixture isdistilled in a higher pressure column operatively associated in a heattransfer relationship with a lower pressure column by acondenser-reboiler configured to condense a higher pressurenitrogen-rich column overhead stream, removed from the higher pressurecolumn, by reboiling an oxygen-rich liquid column bottoms of the lowerpressure column; the first subsidiary compressed air stream and thethird subsidiary compressed air stream, after having been fully cooled,are introduced into the higher pressure column; the liquid air stream isexpanded and introduced into at least one of the higher pressure columnand the lower pressure column; a crude liquid oxygen stream composed ofa liquid column bottoms of the higher pressure column is subcooled,reduced in pressure to that of the lower pressure column and introducedinto the lower pressure column for further refinement; first and secondparts of a high pressure nitrogen-rich liquid stream, formed fromcondensing the higher pressure nitrogen-rich column overhead stream, isused to reflux the higher pressure column and the lower pressure column,respectively; the second of the parts of the higher pressurenitrogen-rich liquid stream is subcooled, reduced in pressure to that ofthe lower pressure column prior to being introduced as the reflux intothe lower pressure column; the crude liquid oxygen stream and the secondof the parts of the higher pressure nitrogen-rich liquid stream aresubcooled through indirect heat exchange with a lower pressurenitrogen-rich column overhead stream withdrawn from the lower pressurecolumn; the at least one liquid stream is one of an oxygen-enrichedstream, composed of the oxygen-rich liquid column bottoms of the lowerpressure column and a third part of the high pressure nitrogen-richliquid stream.
 6. The method of claim 5, wherein: the lower pressurenitrogen-rich column overhead stream is divided into the twonitrogen-rich vapor streams that are utilized to balance the cold endtemperatures of the higher pressure heat exchanger and the lowerpressure heat exchanger; the exhaust stream is introduced into thehigher pressure column; and the liquid air stream is expanded in aliquid expander.
 7. An apparatus for separating a mixture comprisingnitrogen and oxygen, said apparatus comprising: a cryogenicrectification plant configured to compress, purify, cool and distill themixture into nitrogen-rich and oxygen-rich fractions; the cryogenicrectification plant having at least one pump for pumping at least partof a liquid stream composed of one of the nitrogen-rich and oxygen-richfractions in the liquid state, a banked heat exchanger arrangementhaving lower pressure heat exchanger configured to cool at least part ofthe mixture and a higher pressure heat exchanger in flow communicationwith the at least one pump for heating the at least part of the liquidstream and thereby forming a pressurized product stream, means forproducing a boosted pressure stream, means for producing a heat exchangestream, and means for imparting refrigeration into the cryogenicrectification plant; the higher pressure heat exchanger connected to theboosted pressure stream producing means and the heat exchange streamproducing means and configured to partially cool the heat exchangestream by indirectly exchanging heat from the heat exchange stream tothe at least part of the liquid stream, thereby decreasing a warm endtemperature difference within the higher pressure heat exchanger andtherefore, the refrigeration required to be imparted to the cryogenicrectification plant and to cool the boosted pressure stream byindirectly exchanging heat from the boosted pressure stream to the atleast part of the liquid stream; and the lower pressure heat exchangerconnected to the higher pressure heat exchanger and configured tofurther cool the heat exchange stream after having been partially cooledwithin the higher pressure heat exchanger.
 8. The apparatus of claim 7,wherein the cryogenic rectification plant is also configured to generatetwo nitrogen-rich vapor streams and the higher pressure heat exchangerand the lower pressure heat exchanger are also configured to receive andto fully warm the two nitrogen-rich vapor streams so that cold endtemperatures of the higher pressure heat exchanger and the lowerpressure heat exchanger are balanced.
 9. The apparatus of claim 8,wherein: the mixture is air; the cryogenic rectification plant has amain air compressor and a pre-purification unit connected to the mainair compressor to purify the air after having been compressed; theboosted pressure stream producing means comprises a booster compressorconnected to the pre-purification unit; the lower pressure heatexchanger is also connected to the pre-purification unit so that a feedstream composed of the mixture after having been compressed in the mainair compressor and purified in the pre-purification unit is divided intoa first subsidiary compressed air stream that is fully cooled in thelower pressure heat exchanger and a second subsidiary compressed airstream that at least in part is further compressed in booster compressorto form the boosted pressure stream and that and forms a liquid airstream after having been fully cooled within the higher pressure heatexchanger; and the heat exchange stream producing means comprises thehigher pressure heat exchanger also connected to the pre-purificationunit so that the feed stream after having been compressed and purifiedis further divided into a third subsidiary compressed air stream thatforms the heat exchange stream.
 10. The apparatus of claim 9, wherein:the booster compressor is a multi-stage machine; a first part of thesecond subsidiary compressed air stream is discharged from a final stageof the booster compressor and forms the boosted pressure air stream; andthe refrigeration imparting means, at least in part, comprises a furtherbooster compressor connected to an intermediate stage of the boostercompressor to further compress a second part of the second subsidiarycompressed air stream, the higher pressure heat exchanger connected tothe further booster compressor so that the second part of the thirdsubsidiary compressed air stream, after having been further compressed,is partially cooled within the higher pressure heat exchanger, aturboexpander connected to the higher pressure heat exchanger to expandthe first part of the second subsidiary compressed air stream andthereby to produce an exhaust stream and the turboexpander connected tothe distillation column so that the exhaust stream is introduced intothe distillation column.
 11. The apparatus of claim 10, wherein: thecryogenic rectification plant has a higher pressure column and a lowerpressure column to distill the mixture, the higher pressure columnoperatively associated with the lower pressure column in a heat transferrelationship by a condenser-reboiler configured to condense at leastpart of a higher-pressure nitrogen-rich column overhead stream,discharged from the higher pressure column, by reboiling an oxygen-richliquid column bottoms of the lower pressure column; the lower pressureheat exchanger is connected to the higher pressure column so that thefirst subsidiary compressed air stream and the second subsidiarycompressed air stream are introduced into the higher pressure column;the higher pressure heat exchanger is in flow communication with atleast one of the higher pressure column and the lower pressure column sothat the liquid air stream is introduced into at least one of the higherpressure column and the lower pressure column; an expansion devicepositioned between the higher pressure heat exchanger and the at leastone of the higher pressure column and the lower pressure column toexpand the liquid air stream; the higher pressure column connected tothe lower pressure column so that a crude liquid oxygen stream composedof liquid column bottoms of the higher pressure column is introducedinto the lower pressure column so as to be further refined and first andsecond parts of a high pressure nitrogen-rich liquid stream, formed fromcondensing the higher pressure nitrogen-rich overhead stream areintroduced into the higher pressure column and the lower pressurecolumn, respectively, as reflux; a subcooler, positioned between thelower pressure column and the lower pressure heat exchanger orincorporated into the lower pressure heat exchanger, configured tosubcool the crude liquid oxygen stream and the second of the parts ofthe higher pressure nitrogen-rich liquid stream; expansion valveslocated between the subcooler and the lower pressure column to expandthe crude liquid oxygen stream and the second of the parts of the higherpressure nitrogen-rich liquid stream prior to their introduction intothe lower pressure column; the subcooler connected to the lower pressurecolumn so that a lower pressure nitrogen-rich column overhead streamdischarged from the lower pressure column passes in indirect heatexchange with the crude liquid oxygen stream and the second of the partsof the higher pressure nitrogen-rich liquid stream; and the at least oneliquid stream is one of an oxygen-enriched stream, composed of theoxygen-rich liquid column bottoms of the lower pressure column and athird part of the higher pressure nitrogen-rich liquid stream.
 12. Theapparatus of claim 11, wherein: the higher pressure heat exchanger andthe lower pressure heat exchanger are connected to the subcooler so thatthe lower pressure nitrogen-rich column overhead stream divides into thetwo nitrogen-rich vapor streams that are utilized to balance the coldend temperatures of the higher pressure heat exchanger and the lowerpressure heat exchanger; the turboexpander is connected to the higherpressure column so that the exhaust stream is introduced into the higherpressure column; and the expansion device is a liquid expander.